A well-stocked reef aquarium can be a diverse place, full of invertebrate surprises. But identifying the fauna traipsing about on live rock can be a challenge for the average aquarist and few creatures are quite so confusing as the Peanut Worm. In spite of its generic vermiformity (worm shape), this magnificent beast serves an important role in reef ecosystems. So let us sing the praises of this noble worm.
Peanut worms derive their common name from an alleged resemblance to a shelled peanut, though this allusion seems like a bit of a stretch. Scientifically speaking, these organisms are classified into their own phylum (Sipuncula, from Greek “small tube”) which is comprised of a few hundred known species. The true extent of peanut worm diversity is unknown, as this group is poorly studied, especially in the depths of the oceans. Sipunculologists, it would seem, are in short supply.
Being classified as a separate phylum is the apogee of invertebrate originality, indicating a fundamentally unique morphology and evolutionary history. As planktonic juveniles, sipunculans possesses the same ciliated form (called a “trochophore”) found in more familiar groups like mollusks and annelids. This gives clues to the closest relatives of the peanut worms, but the adult morphology is too aberrant to place into either group. Most noteworthy is the presence of a feeding proboscis (the “introvert”) which can be retracted fully into the main portion of the body (the “trunk”). The video below gives a good idea for how this organ operates.
Note that there are other equally obscure invertebrate groups with somewhat similar feeding structures. The penis worms (Priapulida) and ribbon worms (Nemertea) are two taxa with a similar feeding organ, but their introverts are constructed in a different manner from the Sipuncula. Aside from the introvert, there are not many other features of note on a peanut worm. The body is usually covered in a series of microscopic hooks and papillae which are highly useful in species-level identifications. These give the worm a superficially segmented look, but internally there is no repetition of organs as is seen in the segmented worms of the Phylum Annelida.
Peanut worms are rarely colorful, coming in a drab kaleidoscope of greys, oft-whites, browns and creams. Perhaps the most attractive forms can be found in the translucent and opalescent species lurking at the bottom of the ocean. The phylum ranges widely in size, from diminutive species as small as two millimeters (Phascolion) to giant burrowing species which can reach nearly two feet in length (Sipunculus). Some of these larger forms are even collected as a local cuisine in parts of Southeast Asia and Oceania; whether they taste at all like peanuts is not mentioned in the scientific literature.
Coral Reef Sipuncula
Phascolion is a diverse genus (40+ species) of small worms (usually less than 15mm) which has the potential to turn up in aquariums (see the video above). These worms make a living ingesting organic matter from fine sands and silts. They often reside within discarded snail shells (Turbo, Strombus, Cerithium), making these the hermit crabs of the worm world. There are even reports of the worm occurring together with the living mollusk, alongside which can be found a polychaete worm of the family Syllidae intwined around the sipunculan. A crowded home, to be sure.
More interesting still, this species has been found associated with the commensal, free-living coral Heterocyathus. This coral may not be familiar to aquarists, but it lives in the same manner as the more commonly encountered “walking coral” Heteropsammia. Both corals settle as larvae onto snail shells occupied by sipunculan worms. The worm more commonly known from this association is Asipdosiphon muelleri, which differs from Phascolion in having an operculum-like structure located at the base of the introvert.
Juvenile worms search for empty snail shells (usually Cerithium), which are then overgrown by their symbiotic coral. As the coral increases in size, the original snail shell is completely encompassed by the coral’s skeleton, and it becomes incumbent upon the worm to maintain an opening to the outside world. As the worm crawls about in search of greener pastures, the coral goes along for the ride. (They should really be called ‘piggyback corals’). Presumably the worms benefit by a decrease in predation due to the coral camouflage, and the coral benefits by being exposed to a greater diversity of habitats to feed in; Symbiosis is a beautiful thing.
Aspidosiphon (seen in the video above) also has species which have been found burrowed into calcareous rocks (i.e. live rock), as well as inside snail shells (Nassarius). This is accomplished by acidic secretions which dissolve the surrounding limestone, aided by mechanical friction accomplished with the rough papillae along the trunk. I once broke open a piece of live rock which contained numerous specimens of this genus buried several inches inside of it. This is not particularly remarkable, until I tell you that this rock had been regularly soaked in bleach to kill algae. How these worms survived soaking in Chlorox & freshwater for hours on end, year after year, is beyond me.
Themiste is another genus known to burrow into calcareous rocks and coral skeletons. These worms range in size from 10-170mm (most under 30mm) and are a creamy-white or grey color. The trunk occupies most of the length of the animal, while the short introvert is tipped with unusual feathery tentacles. This species is a suspension feeder, using its ciliated tentacles to catch food and move it to the mouth in much the same manner as a feather duster worm might. Other species in this genus have been found in burrows alongside sea grasses, indicating a degree of ecological specialization between species that is apparent throughout the phylum.
The circumtropical Antillesoma antillarum is reported to be one of the most abundant shallow-water, rock-boring peanut worms. It has a similar appearance to Themiste, but differs in having only digitate tentacles (each patterned to form a series of concentric brown rings), giving this worm the appearance of a tiny mop head. The Ringo Starr of the sipunculan world.
The most ubiquitous genus to be found on coral reefs, and, consequently, the most common species found in aquariums, is Phascolosoma. The genus is composed of numerous species, which are best recognized in the aquarium by the pigmented bands patterning the introvert. These can reach nearly a foot in length when fully extended. The specimen in my reef tank appears on occasion during the day to scrape detritus from the rocks surrounding its burrow, but this is undeniably a creature of the night and is more reliably seen in the dark with a well-aimed flashlight. As with other peanut worms, species level identifications are impossible without microscopic examination and dissection.
Most of the remaining genera are exclusively burrowing species that are unlikely to be found in aquariums. Many of these are exceedingly common and diverse groups. Sipunculus (seen in the video below) can be found in huge numbers, and where it can be collected it has been used as both fishing bait, livestock feed, and as a local delicacy. Golfingia, another burrowing genus, contains nearly 100 described species. Interestingly, the scientific name derives from it having been studied between rounds of golf at St. Andrews in Scotland!
The Value of Peanut Worms
You may be asking yourself why sipunculans matter. Of what importance is some obscure little worm from some obscure little corner of the animal kingdom? It is admittedly easy to be underwhelmed by the peanut worm. They are anything but flashy and are the very definition of innocuous. But they are undeniably important creatures to the functioning of healthy marine ecosystems.
Without any real defense mechanism, these soft-bodied creatures are heavily preyed upon and only survive by being incredibly abundant. Upwards of 8,000 worms per square meter have been recorded in some habitats, which means these worms are vital for processing the waste that would otherwise accumulate on the ocean’s floor. They are a crucial link in the food chain… which ultimately benefits species higher up in the pecking order (like us).
Those species which make their living burrowing through rocks indirectly help in creating entire habitats of coral rubble. The burrows of genera like Aspidosiphon and Phascolosoma weaken the reef, which allows for pieces to be broken apart by the action of waves. Areas of accumulated rubble are home to fishes—flasher wrasses and fairy wrasses, for instance—not found on more intact portions of the reef. And every reef hobbyist loves fairy wrasses, so take a moment and give thanks to the humble peanut worm for making it all possible. Three cheers for Sipuncula.